For an in vivo assessment of skeletal muscle development in myostatin deficient mice, we have collected mouse limb muscle from many stages of development and early adulthood. We have determined the skeletal muscle precursor frequency and muscle fiber number in normal and myostatin knockout mice. We are making genetically modified mice to manipulate myostatin signaling in specific cell progenitors. We are also analyzing the role of myostatin in function of muscle stem cells in vitro and in vivo.